Astronomical instruments, such as telescopes, when pointed at an object in the sky (i.e., a star) have a very small field of view. Because the earth is a rotating platform, any object in the field of view of a telescope which rotates with the earth does not stay there but very quickly moves out of the field of view. Therefore, most astronomical devices are rotated mechanically, in a direction opposite to the direction of the earth's rotation, to cancel the effect of the rotation of the earth so that any object in the field of view, or the area scanned by the device, will remain in said field of view or scanned area.
In order to have this mechanical motion accomplish its purpose it is essential that the axis of rotation of the astronomical device be accurately aligned with the axis of rotation of the earth, i.e., that the axis of rotation of the device be parallel to the axis of rotation of the earth, whatever the particular location on the earth's surface of the astronomical device. For instance, if the astronomical device were located at the North Pole (true north, not magnetic north) the axis of rotation of the astronomical device should be vertical, or co-incident with the earth's rotational axis. If the astronomical device were located at the equator of the earth, then the axis of rotation of the astronomical device should be horizontal, pointing in the direction of true north and parallel to (although spaced approximately 4000 miles therefrom) the axis of rotation of the earth. In view of the enormous distances of the stars and other objects in space viewed through telescopes from the earth, this 4000 mile spacing is insignificant and can be neglected in solving the problem which is the subject of the present invention.
At any point between the equator and the North Pole the axis of rotation of the astronomical device would be tilted, i.e., neither vertical nor horizontal with respect to the earth's surface at that point, to an extent corresponding to the latitude at that point, with the direction of pointing again being true north so that the axis of rotation of the device will be parallel to the earth's axis of rotation.
Because it is essential that the astronomical device track accurately the object in space being viewed, even though the device is rotating constantly with the earth, it is necessary that its axis of rotation be very carefully and accurately pointed in the right direction. While it is easy to accurately find the precise latitude of the location where the astronomical device is positioned, it is not easy to point it accurately, or precisely, to true north.
In permanently located instruments this has been accomplished by empirical means, using the drift of stars in the field of view to test the accuracy of pointing. Although this method is effective, it takes a long period of time to accomplish. Therefore, although it may be suitable for permanent installation it is not an acceptable solution to the problem for those installations which are portable, or temporary, or installations that are moved from place to place, or where for any reason quick, as well as accurate, alignment is required.
The precise location of the Celestial Pole (the pole in space through which passes the axis of the earth's rotation) in either hemisphere is not easy to find. There is no bright star located on either pole location.
However, there is in the Northern Hemisphere a star known by the name Polaris (and there are somewhat similar stars in the Southern Hemisphere) which is close to the Celestial Pole, the distance of separation being 49 minutes of arc in the year 1980. The angular distance of Polaris from the Northern Celestial Pole is constantly changing (about three minutes of arc per ten years of time) due to the precession of the earth's axis of rotation. Nevertheless, the location of Polaris relative to the Northern Celestial Pole can be determined, at any particular time, and therefore it can be used as a reference, or base, from which one can accurately locate the Northern Celestial Pole.
It is not satisfactory to use Polaris itself as the theoretical Celestial Pole in any astronomical installation which requires precision in alignment, for example where prolonged viewing or photographing procedures are involved. Although Polaris has been found satisfactory as a reference point or base from which one can accurately locate the Celestial North Pole, Polaris itself cannot satisfactorily be used as if it were, or is assumed to be, the same as the Celestial North Pole.
Nearly all telescopic equipment is provided with an auxillary telescope for pointing the main telescope. The auxillary telescope has a smaller aperture, and a larger field of view, than the main telescope. It is therefore easier to find objects in the sky with auxillary telescopes. Such auxillary telescopes are usually equipped with a reticle for accurately defining the center of the field of view, which is coaxial with the field of view of the main, larger telescope. These auxillary telescopes are known as finders.
Attempts have been made in the past to use finder telescopes to locate the Celestial Pole. To do this, the finder telescope is equipped with a circular reticle, calculated to be the correct diameter for the optics involved to correspond to the circular path of Polaris as it appears to rotate due to the rotation of the earth. If one of these finders, so equipped with a circular reticle, is positioned to be parallel to the axis of rotation of the astronomical device on which it is mounted, then Polaris, when located on the path of said circular reticle, and in the proper position on said path, will serve as a proper reference for aligning the astronomical device with the Celestial Pole. However, this method has disadvantages in that the correct position of Polaris on the circular reticle is hard to judge accurately. The position of Polaris is constantly changing with the diurnal motion of the earth as well as with the precession of the earth's axis of rotation.